1. Field of the Invention
This invention pertains to the medical field and, more particularly, pertains to uniquely designed pacing circuits to allow stimulation of multiple areas of the heart using anodal as well as cathode current.
2. Discussion of the Related Art
Traditional pacing of heart chambers is accomplished by delivering electrical current to cardiac tissue at a cathode of a pacing lead. An anode is provided as either a pacemaker/defibrillator casing (unipolar pacing) or as a separate anode positioned on the pacing lead with a large surface area to prevent anodal capture (bipolar pacing).
Cardiac pacing has been used for at least 50 years to sustain the heart rhythm in patients with slow or absent innate electrical activation. Conventional pacing uses cathodal capture to excite heart muscle. By delivering a current impulse (electrons) via a conductor, a segment of the myocardial cell membrane is rendered more negative such that the threshold potential is reached. This initiates an action potential, which is propagated to adjacent myocardial cells such that eventually the entire heart muscle is depolarized. More recently, pacing of the left ventricle has been used to synchronize the heart in patients with left bundle branch block, thereby reducing instances of heart failure.
Capture of myocardium depends upon current density; a smaller electrode will provide greater current density with the same current than a larger electrode. Traditional theory holds that a certain minimal area of the heart must be captured to allow propagation of the impulse through the heart muscle
Capture also depends upon the ease with which current is transferred to underlying viable myocardial tissue. Diseased tissue or poor connection between electrode and myocardium will increase pacing thresholds. Thus, a small electrode surface area and good contact with viable underlying myocardial tissue is required to achieve capture with minimal current expenditure.
Recent epi-fluorescence membrane studies have demonstrated that the phenomenon of myocardial capture is much more complicated than previously thought. It appears application of current at the cathode, results in the formation of a “virtual cathode” of “dog bone” shape oriented at a 45 degree angle. Stimulation and propagation depend upon the virtual cathode exciting viable myocardial tissue.
Heretofore, anodal capture has been considered undesirable on largely theoretical grounds. In particular, it has been theorized that anodal current may be pro-arrhythmic and may also cause mechanical deterioration of the electrode tip. These theories have now been largely discredited. It has been shown that in left ventricular (LV) pacing, unintended anodal capture occurs frequently and may even have salutary hemodynamic effects.
Anodal pacing occurs when the current introduced to the heart at the cathode returns to the pacemaker circuit via the anode. When the anodal surface area is sufficiently small, this creates an area of hyper-polarization of the myocardial cell membrane. In turn, this sets up a “virtual cathode” remote from the anode. The virtual cathode results in depolarization of the heart in a manner similar to the virtual cathode at the true fixed cathode.
There are at least two advantages to anodal pacing. First, if the virtual cathode lies in an area of healthy myocardium, the anodal threshold may actually be lower than the true cathodal threshold, especially if the cathode lies in an area of diseased myocardium or has a poor connection to the underlying myocardium. Second, if a considerable distance separates the anode and cathode, the area of the myocardium that can be stimulated will be increased by the formation of virtual cathodes. This may have advantages in allowing rapid depolarization of the myocardium; it may also allow capture of a particular small, specifically located structure, such as the His bundle.
The need therefore exists to provide a cardiac pacing apparatus and method including electrical circuits configured to allow for anodal capture.